The generation process of the DPOAE's and the previously used measurement and evaluation methods are described in Dalhoff et al. “Schall- and Geschwindigkeits-DPOAE” [Sound and Velocity DPOAE's] in HNO [ENT] 2010, 58: 543-555, for example. Included there are additional proofs, to which reference is made expressly.
The known methods are used for objectively and quantitatively determining the sound processing in a mammal's ear and thus for examining and subsequently evaluating the faculty of hearing. The methods are based upon the measurement of distortion products of otoacoustic emissions (DPOAE's) that are generated by pairs of excitation signals.
According to EP 2 053 877 A1 and DE 199 05 743 A1, the measurement results can also be used to adjust hearing aids.
The auditory system can be regarded as a chain of processing blocks, which are gone through before the more complex auditory perception takes place in the cortex. The first blocks are the outer ear (auricle and acoustic meatus), the middle ear (auditory ossicles with the base plate as boundary to the fluids of the inner ear), and the fluid-filled inner ear. These three blocks are also called the periphery; several neural processing nodes follow these blocks, before the signals arrive in the cortex. The inner ear comprises the cochlea, which constitutes the receptor field for the auditory perception and in which sounds are decomposed into their individual frequencies in a manner similar to a Fourier analysis.
Most hearing impairments develop in the inner ear. This also includes the age-related hearing loss, which on average results in a 35 dB hearing loss in men 60 to 70 years old and in a 25 dB hearing loss in women of the same age group at frequencies starting at 4 kHz.
This age-related hearing loss is dominated by an impairment of the so-called cochlear amplifier (the mechanical amplification of the traveling wave in the cochlea) in the inner ear, which amplifier in the healthy condition achieves an amplification of the vibration in the inner ear by the factor 300 to 1000 through a complex interplay of electro-mechano-biochemical mechanisms, before the vibrations are converted into neural signals by the inner hair cells.
The key element of the cochlear amplifier is constituted by the outer hair cells, which in principle act like piezo actuators. In contrast to most impairments of the middle ear, impairments of the cochlear amplifier cannot yet be treated successfully today.
The condition of the middle ear can generally be detected sufficiently by means of tympanometry. The condition of the entire auditory system is examined subjectively by pure-tone audiometry and speech comprehension tests, and objectively by diversion of neural excitation.
In case of a hearing loss, if an impairment of the middle ear components is excluded, the decision between a neural and cochlear impairment remains. The measurement of otoacoustic emissions (OAE's) is used for this purpose. OAE's are active, acoustic emissions of the ear that retrogradely, i.e., opposite to the direction of sound perception, arrive via the path through the auditory ossicles and the eardrum in the acoustic meatus and can be recorded there by means of highly sensitive measuring microphones.
Two different types of OAE's are distinguished, viz., the spontaneous OAE's and the evoked OAE's, which are evoked by acoustic stimuli. The spontaneous OAE's (SOAE's) occur in 35 to 50% of healthy ears and are not audible to the producer himself, and they do not have any substantial clinical significance.
The evoked OAE's (EOAE's) occur during or shortly after an acoustic stimulation of the ear. Depending upon the form of the acoustic stimulus, different subgroups of evoked OAE's are distinguished, which, in particular, include the transitory evoked OAE's (TEOAE's), which are detectable after a short acoustic stimulus, and the distortion product otoacoustic emissions (DPOAE's), which are generated by two simultaneously applied sinusoidal tones (f1 and f2).
The method according to the invention for determining the condition of the cochlea is based upon the measurement of the DPOAE's, i.e., the subgroup of the OAE's that has been researched particularly intensively.
DPOAE's are byproducts of a healthy, active inner ear amplifier, which amplifies the vibrations generated in the inner ear by an acoustic stimulus by a factor of 100 to 1000 in humans and other mammals, before the conversion into neural signals occurs. For this purpose, the cochlear amplifier uses external energy that must be provided by the metabolism.
DPOAE's are generated by simultaneous stimulation with two primary tones f1 and f2 with excitation levels L1 and L2. In this case, the two primary tones f1 and f2 are so-called “pure tones,” which contain precisely one frequency only. In accordance with the general Fourier relation between the time and frequency domain, these tones would therefore have to continue infinitely, since the spectrum would otherwise widen. Since this infinitely long continuation cannot be realized, the person skilled in the art considers these to be tones that are presented for so long as their spectrum is sharp. The marked non-linearity in the characteristic curve of the cochlear amplifier results in the generation of distortion products, which are partially transmitted retrogradely back into the acoustic meatus and can be measured there using suitable instruments. In the process, a certain distortion product is evaluated, which is preferably at the frequency fdp=2f1−f2. Its amplitude allows conclusions regarding the condition of the cochlear amplifier, which conclusions are useful in the general clinical practice, e.g., in the screening of newborns for hearing impairments requiring treatment.
Generally, the amplitude of the DPOAE is extracted from the spectrum of the measured signal using Fourier transformation. Since the DPOAE's have a very low sound level, which can be considerably below the hearing threshold, averaging must be performed long enough in order to obtain a certain signal-to-noise ratio and thus reliable diagnostic information.
In diagnostic applications, the frequency ratio f2/f1 is preferably held constant, because each species has a frequency ratio where the DPOAE is strongest and can thus be measured most easily. In humans, this ratio is 1.2 and thus corresponds to a minor third.
The main part of the DPOAE signal is produced in the inner ear at a location where f2 is actually mapped. This is because the traveling wave of f2 is there at a maximum, and the traveling wave of the f1 tone is also already very strong at that location. On the other hand, at a further apical location of the maximum of the f1 traveling wave, the f2 wave is already completely collapsed. The f2 mapping location is therefore the location where both tones are relatively strong and are thus processed simultaneously by the strongly non-linear characteristic curves of the ion channels of the outer hair cells.
The person skilled in the art, as well as the ENT physician, therefore always associates a DPOAE event with the f2 frequency, i.e., compares a DPOAE stimulated with f2=3 kHz with the audiogram at 3 kHz, even though the frequency of the DPOAE itself is below that by, fairly precisely, two thirds.
If several such DPOAE's are measured at a frequency f2 and at different sound levels L2 and combined in a so-called growth function, more precise information about the function of the cochlear amplifier in the inner ear results. For each frequency f2, the so-called threshold value can then be determined using the growth curve, which threshold value is the lowest excitation level L2 at which the DPOAE still reaches a given minimum signal-to-noise ratio. This threshold value cannot be measured, since the noise measured along with it is finite, but must be determined by extrapolation.
In order to obtain diagnostic information across the entire frequency domain, 6 to 8 growth functions are therefore typically measured sequentially.
The growth curves determined in this way and the threshold values extrapolated from them can then be used as the basis for improved diagnostics and to adjust hearing aids, because the extrapolated threshold values can be regarded as a direct statement about the hearing loss; see DE 199 05 743 A1 mentioned above.
In this case, the excitation of the DPOAE's can be carried out both by continuous tones and by pulsed tones. As already mentioned above, continuous tones in this respect refer to tones that are presented for as long a time as their spectrum is sharp. In case of the pulsed DPOAE's, f1 is fed in as a continuous tone or in a pulsed manner, and f2 is fed in in a pulsed manner, wherein the ratio of L2 to L1 is adjusted to a certain range, and L2 is then changed gradually. According to the general Fourier relation between the time and frequency domain, the pulsed tones are tones the spectrum of which is widened as a result of the shortness of the pulse. If one of the tones, such as the aforementioned tone f1, is presented as a continuous tone, this means that the tone f2 fed in in a pulsed manner passes through an on-and-off process while the tone f1 continues.
By measuring the growth functions using pulsed DPOAE's, certain artifacts, known as the “two-source problem” in continuous tone DPOAE's, are prevented.
The method is based upon two key components: 1) Extraction of the so-called non-linear components of the DPOAE's using pulsed stimulation and temporal isolation; 2) Measurement of the growth functions of pulsed DPOAE's using the extrapolation method of Boege & Janssen, with the important modification of the “high-level saturation correction.” Both methods are described in Dalhoff et al., “Two-source interference as the major reason for auditory-threshold estimation error based on DPOAE input-output functions in normal-hearing subjects,” in Hearing Research 296 (2013), pp. 67-82.
The present invention addresses the improvement of this method in detail.
The known method has a disadvantage that prevents it from being used in the ENT routine. The estimation of the auditory threshold at only one frequency takes about 480 s with the known methods, wherein obvious measures to reduce this measurement period to 96 s were discussed. If the usual set of seven frequencies for the clinical description of the faculty of hearing is to be tested, a measurement period of 11 min results even for those with normal hearing.
A substantial disadvantage of the known method thus consists in the fact that it is very slow, and that it was therefore considered so far to not be usable clinically—in particular, because it is far too lengthy for examining the hearing-impaired. Moreover, the achievable accuracy in the determination of the growth curves and the extrapolated threshold values is often not satisfactory.
This is the point from which the further developed method according to the invention of fast pulse distortion product otoacoustic emissions (pulse DPOAE's) proceeds, the task of which invention is to create a quickly performed and nonetheless precise method of the aforementioned type.